This proposal seeks support for studies to determine how testicular androgen regulates the differentiation early in life of neural tissues destined to control sexual behavior in the adult male ferret. Previous research suggested that in ferrets, as in rhesus monkeys, a perinatal period exists during which exposure to androgen causes behavioral masculinization. However, in neither species has perinatal period been identified during androgenization causes behavioral defeminization. Experiments are proposed which will search further for defeminizing effects of perinatally administered steroids in ferrets, and which will explore the possible contribution of two metabolites of testosterone, estradiol and 5 alpha-dihydrotestosterone (DHT), to the differentiation of sexual behavior. These studies will: 1) measure plasma levels of testosterone in male and female ferrets just prior to and after birth, 2) attempt to induce behavioral defeminization in female ferrets by giving testosterone during perinatal periods other than those which have previously been studied, 3) compare the ability of testosterone, estradiol, DHT, and synthetic analogues of these steroids to cause behavioral masculinization when given neonatally to female ferrets, and the ability of various drugs which inhibit the formation or neural binding of estradiol or androgen to block masculinization in males, and 4) determine whether estrogen or androgen binding molecules are present neonatally in cytosol fractions of ferret hypothalamus.